The current C implementations **crash** if the input includes a surrogate
Unicode code point, which is not possible to encode in UTF-8.

Important notes:

1.  It is possible to pass a non-UTF-8 string as a separator to the
    `.isoformat()` methods.
2.  The pure-Python `datetime.fromisoformat()` implementation accepts
    strings with a surrogate as the separator.

In `datetime.fromisoformat()`, in the special case of non-UTF-8 separators,
this implementation will take a performance hit by making a copy of the
input string and replacing the separator with 'T'.
Co-authored-by: Alexey Izbyshev <izbyshev@ispras.ru>
Co-authored-by: Paul Ganssle <paul@ganssle.io>